1. Field of the Invention
The present invention relates to a method of calibration for analogue probes. The method has particular reference to the calibration of analogue probes which have a stylus for contacting a workpiece, and which is mounted on a mechanical suspension, for example a spring suspension.
2. Description of the Related Art
Analogue probes of this type are well known and an example of such a probe is described in our UK Patent No. 1,551,218. This patent describes a probe suspension mechanism which comprises three orthogonally arranged pairs of parallel springs connected in series between a fixed point on the probe housing and a movable member to which a workpiece contacting stylus is connected.
During a measuring operation on a workpiece using such a probe, a machine on which the probe is mounted is driven towards the workpiece to bring the stylus into contact with the workpiece surface at various points on the surface. When the stylus contacts the workpiece the stylus will be deflected as the machine continues to move, and measuring transducers within the probe generate outputs representing deflections of the probe stylus along three orthogonal axes. These axes are referred to as the a, b and c axes of the probe.
Ideally it would be arranged that the a, b, and c axes of the probe are aligned with the X, Y and Z coordinate axes of the machine when the probe is mounted on the machine, so that the measured deflections of the probe stylus will take place along the X, Y and Z axes of the machine. However, such alignment is not always possible to achieve.
Also, if there is any mis-alignment between the three probe a, b and c axes, such that they are not orthogonal, then deflection of the stylus, for example, nominally in the a direction can give rise to deflections in the b and c directions also.
Additionally, the scaling factors of the three probe axes will, in general, deviate from their nominal values.
Therefore, it is usual to calibrate the probe and machine system to determine the effects of any such mis-alignments and scaling errors, and thereafter to correct any measurements made on a workpiece for these effects.
One method of performing the calibration which is described in our International Patent Application No. WO00/25087 is to mount a calibration artefact (usually a reference sphere of known diameter) on the machine, and to drive the probe towards the artefact, for example, along one of the machine axes, until an increase in the output of the measuring devices of the probe above a pre-determined threshold level indicates that contact with the surface of the artefact has been made. After stylus contact has been confirmed, a set of machine X, Y, Z and probe a, b, c coordinate data are taken. Machine movement continues until the machine has moved a selected distance beyond the confirmed contact point, and a further set of X, Y, Z, and a, b, c coordinate data are taken.
The changes in the a, b, c outputs of the probe's measuring transducers in the three axes are recorded and correlated with the changes in the readings of the machine's measurement devices along each of the three machine axes. This procedure is repeated for two other orthogonal directions, which may be the other two machine axes, and from the sets of readings a probe transformation matrix can be established which relates the probe outputs in the a, b and c axes to the machine's X, Y and Z coordinate system. This involves solving the nine simultaneous equations relating the a, b, and c axis data to each of the X, Y, and Z axes. This process may be repeated for one or more further deflections but normally only relatively few data points are taken.
Once the transformation matrix has been established the relevant machine axis components of the probe deflections can be obtained by multiplying the relevant probe output by the relevant matrix term.
The key assumption in this calibration is that the machine movement mirrors the movement of the stylus tip relative to the probe. However, this assumption becomes invalid when the stylus slips on the surface of the sphere.
There are two factors which can cause the stylus to slip on the sphere surface;    i) the machine may not go down the commanded direction accurately enough to prevent slippage,    ii) the probe force and deflection vectors may not coincide closely enough to prevent slippage.
Although the sensitivities of the probe axes are accurately determined by this method, side slip of the stylus generates false directions for the probe axes.
Furthermore, when scanning with the calibrated probe, tip friction drag causes a significant component of probe displacement in the negative scan direction.
The combination of the false directions of the probe axes mentioned above with tip friction drag causes an error in the apparent material condition of the surface (i.e. in the direction normal to the surface).